The present invention relates in general to phase frequency detection circuits and, more particularly, to a phase detector having an extended linear operating range.
Most if not all wireless communication systems transmit and receive information on a modulated carrier frequency. The radio frequency (RF) signal transmitted across the air-ways is a high frequency carrier modulated by the signal level of the transmitted information. In a cordless telephone, for example, the voice and/or data to be transmitted is modulated and frequency up-converted for transmission by a transmitter section. The received voice and/or data is frequency down-converted and demodulated in a receiver section to hear in the speaker or see in the phone display.
The transmitter and receiver sections typically use synthesizers for the frequency conversion and modulation processes. The synthesizers include a phase detector, operating within a phase locked loop (PLL), to control an oscillator frequency used for the frequency conversion and modulation. The phase detector compares the phase difference between a reference frequency and the oscillator frequency and produces an error signal representative of the phase difference. The phase detector should operate in a linear range in terms of error signal versus phase difference. In the prior art, the phase detector becomes non-linear when the phase difference exceeds .+-.2.pi., which occurs when the reference frequency and oscillator frequency are substantially different. The non-linear region of the phase detector prevents the PLL from readily re-acquiring phase lock if the phase between the reference frequency and oscillator frequency become significantly different.
Hence, a need exists to extend the linear operating range of a phase detector beyond a phase difference of .+-.2.pi..